Conventional drilling techniques in the fields of Oil & Gas, Mining, Geothermal Energy, Civil Engineering and other activities consist of rotating a drilling tool downhole and at the same time applying a thrust force to it in the order of a few tons to several tens of tons. Rotation of the drilling tool is provided by rotation of the entire drill string from the surface (a system called “rotary drilling” in the art) or using a bottom hydraulic motor (turbodrilling). The drilling tools used are the tricone wheel type, PDC (Polycrystalline Diamond Compact) or impregnated matrix. In all cases, destruction of the rock is produced by mechanical effect. Rock cuttings produced by the tool are raised to the surface in the space between the walls of the hole and the drill string (the annulus) through the upward flow of drilling fluid.
However, these techniques suffer from slow forward progress in certain very hard or very abrasive geological formations. To resolve this problem, various alternatives to conventional techniques have been devised. Among these various attempts, a technique has been proposed based on repetitive injection of very high power electrical impulses directly into the ground through electrodes placed under the drilling tool. Electric arcs are produced between electrodes, penetrating the ground and creating a plasma tunnel. The expansion of gasses generated by the plasma fractures the rock and produces cuttings which are then eliminated in the conventional manner by fluid flow. This technique, well known for a long time, has different names in the literature such as “drilling be electrical discharge pulses”, “plasma channel drilling process” or “pulsed electric rock drilling apparatus”.
Document US005845854A, referring to previous publications, shows how to optimise the inter-electrode distance based on the voltage rise time. Document U.S. Pat. No. 6,164,388 gives equations to optimise operation and claims an optimised power circuit design using semiconductor rectifiers. Document WO-A-03/069110 provides orders of magnitude relative to the electrical parameters of this process (voltage, power, pulse duration). However, these three patents suffer from a major weakness, namely the supply of electric power to the electrodes. Indeed, the pulse generator for these system is located at the surface. A means of transmission (by cable or other system) is therefore necessary to connect the surface to the borehole bottom, which leads to complexity and safety concerns.
Certain documents highlight the combination of this technique with other processes. Thus, document U.S. Pat. No. 7,416,032 refers to a system for drilling by electrical discharge with a combination of electrical and mechanical effects. Document U.S. Pat. No. 7,527,108 refers to a portable system for drilling by electrical discharge in a mining context for linear metric boreholes. Document U.S. Pat. No. 7,784,563 refers to a system for drilling by electrical discharge, including a mechanism to maintain continuous contact between the rock and the electrodes. Document EP2554780 presents a combination of a system for drilling by electrical discharge and a process for cooling and pulsation of the drilling fluid. Document EP2554778 presents a combination of a system for drilling by electrical discharge, a system of sensors for directional drilling and a LWD (Logging While Drilling) system.
All these documents present the same weakness: despite the presence of the pulse generator at the bottom of the borehole, the electric power required to supply it is provided by a cable from the surface. However, the presence of a cable is a major obstacle which conflicts with the operational use of these systems. Indeed, in the case of using conventional drill strings, the presence of a cable prevents their rotation. Such a handicap contradicts a fundamental rule of the profession: to be able at any moment to turn the rod casing.
Certain documents however suggest the possibility of using a downhole electricity generator to power a system for drilling by electrical discharge, such as for example documents US2009/00500371, U.S. Pat. Nos. 8,109,345 and 7,784,563. However, these documents provide no details on the operation of the system in such a configuration, the first document being only on a non-rotating system. However, one of the main advantages of a downhole electricity generator is the ability to turn the drill string from the surface. In addition, for using a downhole electricity generator, these documents do not address the following three essential questions: control of the systems' operation from the surface, the safety of personnel in relation to the risk of high voltage when lifting out the drill string and compatibility with the use of a MWD (Measurement While Drilling) system which is almost routine these days, especially for oil drilling.